This invention relates to the field of ultrasonic transducers for use with probes. Examples are transducers associated with catheter tips for use with cardiac pacing and electrophysiologic study leads that are implanted temporarily or permanently within a body, and needle tip transducers useful for biopsy and catheterization. The invention also relates to the probes themselves and methods for their manufacture.
External and implanted cardiac pacemakers are widely used to diagnose and treat a broad class of cardiac arrhythmias. The electrical pulses for heart stimulation are applied to the heart muscle via flexible leads with active electrodes on their tips. Electrical signals of the heart can be measured using the same technique with multipolar leads for electrophysiologic studies.
Cardiac catheterization requires the accurate positioning of a catheter within the heart. Positioning is commonly accomplished through the use of an x-ray system. Recently there has been interest in the use of two-dimensional ultrasonic echocardiography to localize the catheter tip. Usually an ultrasonic acoustic transducer is mounted at the tip of the catheter to allow location of catheter tip employing standard ultrasonic seeming equipment. Such equipment operates by employing a transducer to detect ultrasonic radiation emitted from the equipment and reflected from tissue under study. It is also known to emit ultrasonic energy from other sources and detect it at the equipment transducer. As the catheter is inserted into the patient's body and into the scan plane of an external ultrasound imaging probe, the ultrasound energy is detected by the catheter tip transducer. Based on the elapsed time between transmission of a pulse from the imaging probe and the reception of an echo by the catheter tip transducer, it is possible to calculate and display the exact position of the tip of the catheter on the display unit of the imaging probe.
A convenient way to ensure that the catheter tip transducer is omnidirectional is to use a tubular shaped transducer mounted on the wall and near the tip of a catheter. A tubular-shaped transducer has an omnidirectional radiation pattern in the plane perpendicular to the axis of the transducer. Piezoelectric polymer film is often used as transducer material because it is thin, flexible and can be made into complex shapes. Common types of piezoelectric film are PVDF or PVDF co-polymer. Use of such films has, however, encountered manufacturing difficulties since the polymer film is difficult to roll into tubular form because of the small dimension of the film. Typically, the edge of the film tends to lift up. Further, it is difficult to ensure that the adhesive material is uniformly distributed between the catheter surface and the PVDF film.
In various biopsy and catheterization procedures, accurate localization of the tip of the biopsy needle or a catheter is required. A number of methods have been proposed that involve the use of ultrasound imaging for locating the tip.
Basically this requires mounting one or more transducers on the tip of the instrument. By using an ultrasound imaging probe, the patient's body is examined to locate the point of interest and guide the instrument tip to that position.
Jan Lesny and Joseph Aindow (UK Patent GB2 157 828 issued on Mar. 4, 1987) described a method to mount a transducer at the end of a needle tip. In this invention, a small PVDF element is mounted on the central conductor by means of a small dab of silver loaded epoxy. A thin film of varnish is then applied to the edge of the element to ensure electrical isolation of the front and back faces of the PVDF element. A thin film of gold is then sputtered over the entire outer surface of the needle to provide electrical conductivity. A protective layer of one or more metal layers is then electroplated over the sputtered layer.
There are several difficulties with this design. Firstly, the PVDF element mounted on the central conductor is very small (on the order of 100 micron), therefore it is very difficult to handle and position accurately on the tip of the needle. Secondly, it is very difficult to use a dab of silver loaded epoxy to connect to one electrode of the PVDF film (the back electrode) and at the same time ensure that the epoxy is not smeared or contact is made with the other electrode of the PVDF film (the front electrode). Contact with both the front and back electrodes by the silver loaded epoxy causes a short in the PVDF film and renders it useless. This is a severe problem because the PVDF film is very thin, in the order of 8-30 micron thick.
Vilkomerson (U.S. Pat. No. 4,407,294 issued Oct. 4, 1987) described a method to determine the location of a needle tip by mounting a number of hemispherical transducers on the opposite sides of the tip of an elongated stylet. Again, this technique is extremely difficult because of the need to mount the hemispherical transducers onto the tip of the stylet.